How to create a cube from two planes?
Based on the information given, I think that the loop tools addon should help you. If you don't have it enabled then go to your user preferences and enable it. 

This will require both planes to be a single object, so start by joining them with Ctrl+J.  

Now, in the tool shelf, go to the loop tools section and with both opposing faces selected (which will be the inside of the 'cube') click the bridge tool (highlighted in yellow). If you have missing faces after the bridge operation then tick the remove faces option (highlighted in red) so it is disabled and the faces will come back.